The present invention relates to a toner for use in a developing process such as an electrophotographic process, electrostatic recording process and electrostatic printing process, and more particularly to a black toner or a color toner suitable for use in an oil-less fixing system.
In image forming apparatuses, such as copying machines and laser printers, which adopt the electrophotography, the electrostatic recording method or the electrostatic printing method, a toner is used as a developer for rendering an electrostatic latent image a visible image. In a step of fixing the resultant toner image onto a receiving material such as paper, generally adopted is a method wherein heat and/or pressure is applied to the toner image and a surface of the receiving material by means of a fixing roller (heat roller).
Since the fixing roller contacts the toner image in a molten state, the fixing roller is likely to encounter a so-called hot offset phenomenon wherein a part of the toner image is adhered or transferred to a surface of the fixing roller and re-transferred to another receiving material. The hot offset phenomenon leads to the contamination of formed images and hence, it is a general practice to carry out the fixing process with the fixing roller coated with oil for enhancing toner releasability from the fixing roller.
More recently, a technique for forming a color image such as a color copy has come into wide use, the technique wherein toner images of different colors are transferred and fixed to a single receiving material thereby forming a color image thereon. Unfortunately, a color image formed with color toners, in particular, tends to have a glossy image surface (so-called shine) which results from the oil adhered to the toner, the oil applied to the fixing roller surface. This makes it difficult to attain a satisfactory image quality of the color image. There is another problem that while recent years have seen an increasing demand for adding a retouch characteristic to the formed images, the retouch characteristic is impaired by the oil adhesion to the toner.
On this account, it is desired to adopt the so-called oil-less fixing system wherein the fixing roller is not coated with oil. In this case, however, the toner must be increased in the releasability from the fixing roller in order to prevent the hot offset phenomenon.
Conventionally, a method of admixing wax in toner particles in high concentrations has been adopted as means for increasing the releasability of the toner from the fixing roller (see Patent Documents 1 and 2). However, the method tends to encounter a phenomenon wherein the wax separates from the toner particles to adhere to carrier, thus entailing image degradation.
Patent Document 1: Japanese Unexamined Patent Publication No.2001-296692 (Paragraphs [0043] to [0044])
Patent Document 2: Japanese Unexamined Patent Publication No.2001-296693 (Paragraphs [0044] to [0045])